The entire article goes into much more detail, but here's a quick sanity check.

So, I’m not particularly worried that Treasury bonds are a bubble about to pop. Here’s why:

The simplest reason to be skeptical of the bond bubble story? A bubble means that people are buying an asset at ever-rising prices for speculative reasons, not for the fundamental value of the asset but because they are assuming somebody else will buy it at a higher price. I see no evidence of this behavior by buyers of Treasury bonds. They tend to be people looking to get their 2 percent yield and their money back at maturity. And while they may complain that the yield is so low (and the price of the bond, by extension, so high), that they are buying the bond because the guaranteed 2 percent is a better deal than the alternatives — not because they’re convinced that interest rates will fall further and they’ll be able to “flip” the bond at a profit.